Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 55

53
The fact that there are no Table mountains either, in the area
under consideration, supports, on one side, the conclusions in
(8), while after our discussions in Chapter 2, it is quite un-
necessary to say that it also refutes the hypothesis of a sub-
glacial origin of Table mountains.
We have sofar only considered the correlation between the
severe climate periods and the volcanism in Iceland. In each
case the volcanic expression was a shieldvolcano.
But the correlation of climate and activity is clear all through
the postglacial and historic time.
We shall first discuss the latter, as in that way we recognize
that the three volcanic zones in the country are different as
to sensitivity to the climatic fluctuations, and we are able to
see how slight variations in the climate find an expression in
volcanic activity.
In the prehistoric postglacial time, on the other hand, we
have mainly to deal with climatic periods of about 1000 years’
duration, and can only see the correlation between clustering
of eruptions within such long periods and the general character
of the climate. But just this will show us, what was stressed
in connection with the shieldvolcanoes, at the end of Chapter 1,
that periods with predominant easterly winds, are the vol-
canically active ones. For most of the historic time, it is more
difficult to decide upon this factor.
In Fig. 7 is reproduced the simplified diagram by Sigurður
Þórarinsson, showing the variations of a generalized annual
temperature in Iceland from about 800 A.D. to the present
time. The diagram is based on temperature measurement back
to 1846, the changes of the extension of glaciers, dated tephro-
chronologically or by historical evidence. Further, such factors
as the cultivation of corn in the earliest centuries are used.1)
1) We should add that the Deuterium percentage in an icecore from
Bárðarbunga, reaching back to 1550 A.D. is a good indication of air
temperature, and agrees well with Fig. 6. See Ámason, Bragi, 1976:
Groundwater Systems in Iceland traced by Deuterium. Soc. Sci. Isl. No. 42,
p. 1—236 (Fig. 13).